Add threat warning as a function

[iMMAP-GSM]

Threat Warnings will be used as a quick way to inform relevant groups of staff of a current situation which might cause an imminent threat.

The functionality is similar to Security Advisories and contains the same input fields, but geo referencing / tagging will need to be added. A separate icon for threat warnings will have to be developed.

The functionality needs to include a "send sms" and "send email" option (see Issue: Add "send email" option)